Process for manufacturing a part made of an ai/ai3b48c2 composite material

ABSTRACT

A method for manufacturing a part made from an Al/Al 3 B 48 C 2  composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al 3 B 48 C 2  are dispersed. The method comprises the following steps: a) placing a powder of chemical formula AlB 2  in the cavity of a graphite crucible; b) closing the cavity by use of a graphite element; c) heating the crucible to a temperature of at least 960° C. and less than or equal to 1400° C. in order to obtain the formation of precipitates of the mixed carbide of chemical formula Al 3 B 48 C 2  in liquid aluminium; d) cooling the crucible in order to solidify the liquid aluminium; e) removing the crucible; thereby the part made from Al/Al 3 B 48 C 2  composite material is obtained.

TECHNICAL FIELD

The present invention relates to the field of the synthesis of composite materials having a metal matrix and ceramic particulate reinforcements.

In particular, the invention relates to a method for producing a composite material having an aluminium matrix in which Al₃B₄₈C₂ reinforcements are dispersed.

This method may in particular find an application in the aeronautical and automobile fields.

STATE OF PRIOR ART

In the automobile and aeronautical field, manufacturers seek to obtain lightweight strong materials. However, the majority of lightweight materials that can be used industrially are not very strong.

This is the reason why materials with a metal matrix (CMMs), which comprise a metallic matrix (metal or metallic alloy) in which metallic or ceramic reinforcements (particles, fibres or the like) are incorporated, are particularly appreciated. This is because the advantage of CMMs compared with light alloys (based on aluminium, magnesium or titanium) is that they have very high E/ρ (modulus of elasticity/density) and σ/ρ (elastic limit/density) ratios.

One of the main problems to be solved when producing a metallic-matrix material is that of the chemical reactivity between the matrix and the reinforcement. This reactivity is in fact necessary for the interface between the matrix and the reinforcement to be mechanically strong, but it may also lead to catastrophic secondary effects for the composite material. This reactivity must therefore be strictly controlled. Let us take the example of the composite material Al/B₄C, which is a composite material that is particularly advantageous because boron carbide is one of the hardest known materials, is lightweight and has a melting point above 2400° C.

The chemical reactivity between the Al phase and the B₄C phase of the Al/B₄C composite is necessary since the liquid aluminium does not wet the B₄C particles, which involves a difficulty in providing, in the composite material, an intimate interface between the two phases. Consequently the adhesion work, namely the chemical force of the interface, will be weak in the composite material, which leads to a mechanically weak interface. However, in the majority of composites with a metallic matrix and ceramic reinforcement, there is an objective of transfer of the mechanical load from the matrix to the reinforcement through the interface: the latter must therefore be mechanically strong.

However, the chemical reactivity must also be controlled. This is because the equilibriums between the phases in the Al—B—C system indicate that Al and B₄C are not in equilibrium below a temperature that is not precisely known, but it is estimated in the literature as being above 1400° C.

Thus any synthesis based on the mixing of powders of precursors of Al and B₄C and carried out at a temperature below 1400° C. therefore leads to the decomposition of the B₄C reinforcement by Al and to the formation of the carbide Al₃BC₃. The latter, although much less sensitive to hydrolysis in the presence of moisture than the Al₄C₃ carbide, all the same remains subject to this phenomenon, which leads to the release of large quantities of gaseous CH₄. The gas produced at the core of the composite material then causes stresses that lead to the ruin of the composite material (return to the powder state). Moreover, other phases such as the borides AlB₂ and AlB₁₂ may also be formed during the interaction between Al and B₄C. The fragility of these phases then causes weakening of the composite material.

A synthesis carried out at high temperature (at least greater than 1400° C.) in the domain where Al and B₄C are in equilibrium would for its part be faced with the same difficulty, but this time during cooling, then leading to the same consequences.

The solutions envisaged in the literature for producing an Al/B₄C composite aim to solve the problem of reactivity by acting on the reaction kinetics, either by working at very low (cryogenic) temperature for the purpose of slowing the reaction kinetics between the Al and B₄C to the maximum extent, or by limiting the reaction time at high temperature during the consolidation/formation step. However, these solutions are not suited to industrial production.

The first solution, which will be termed “cryogenic method”, was developed by Julie Schoenung at the University of California at Davis. This method comes up against the difficulty of using high-energy grinding in liquid nitrogen for large quantities of material. The change from laboratory scale to industrial production appears to be difficult. Moreover, this method cannot avoid a consolidation step, which must be carried out hot.

The second solution consists of minimising the duration of the hot consolidation step in order to limit as far as possible the progress of the reaction between Al and B₄C. The main difficulty lies there also in the quantity of material that can be used. This is because hot formation requires the Al matrix to be raised to a sufficient temperature to be subject to plastic deformation by creep. However, in the case of a large volume of material, making the temperature uniform throughout the whole of the volume requires also a long temperature-maintenance time.

A third solution combining these two approaches was proposed by the team of Julie Schoenung and was the subject of a patent application (document [1]). It consists of mixing and grinding powders of precursors of Al and B₄C in liquid nitrogen (cryogrinding), compacting the mixture cold, and then sintering the compacted mixture by the SPS (spark plasma sintering) technique, known as flash sintering, which makes it possible to raise the compacted mixture to high temperature for a shorter time than with conventional heating techniques. This third solution does not however solve the problem of change of scale for the cryogrinding step.

Thus, because of the reactivity between Al and B₄C, the conventional methods consisting of mixing, compacting and densifying the powders are not satisfactory, unless they are used in the context of cryogenic techniques. However, such cryogenic techniques are difficult and expensive to implement and are not suited to a production of a large volume of material.

The inventors therefore set out to design a method for producing a composite material alternative to the Al/B₄C composite that has properties similar to those of the Al/B₄C composite, while being able to be carried out industrially.

DISCLOSURE OF THE INVENTION

This aim is achieved by means of a method for manufacturing a part made from an Al/Al₃B₄₈C₂ composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al₃B₄₈C₂ are dispersed, said method comprising the following steps:

-   -   a) placing a powder of chemical formula AlB₂ in the cavity of a         graphite crucible;     -   b) closing the cavity by means of a graphite element;     -   c) heating the crucible to a temperature of at least 960° C. and         less than or equal to 1400° C. in order to obtain the formation         of precipitates of the mixed carbide of chemical formula         Al₃B₄₈C₂ in liquid aluminium;     -   d) cooling the crucible in order to solidify the liquid         aluminium;     -   e) removing the crucible;     -   thereby the part made from Al/Al₃B₄₈C₂ composite material is         obtained. Preferably, the graphite element used to close the         cavity is a graphite piston. When it is placed in the graphite         crucible at step a), the AlB₂ powder may be in various forms.         According to a first variant, the powder is placed in the         crucible in a compressed form, for example in the form of one or         more pellets. According to a second variant, the powder is         placed in the crucible in a powdery form and step b) in addition         comprises a compression of the powder. It is preferred to use         the powder in powdery form and to compress it in the cavity of         the crucible, since, diboride AlB₂ being not very ductile, it is         difficult to obtain compaction.

According to a preferred variant of the invention, when the powder is placed in the crucible in powdery form, step b) further comprises the compression of the powder. Preferably, the compression of the powder and the closure of the cavity of the crucible are obtained by using a graphite piston. The piston is sized so as to be able to slide in the opening in the crucible in order to compress the powder and close off this opening.

Preferably, at step c), the crucible is heated to a temperature ranging from 1000° C. to 1400° C. for a period ranging from 5 minutes to 30 minutes. Preferably, the temperature drop at step d) is rapid. This makes it possible to limit the decomposition reactions of the phases formed at high temperature. Preferentially, the cooling at step d) comprises a temperature drop with a rate greater than or equal to 10° C./second until 600° C. is reached.

The removal of the crucible at step e) may be obtained by separating the ingot of composite material obtained at the end of step d) (and which forms the composite material part to be obtained) from the crucible or by carrying out a turning operation that will destroy the crucible.

The Al/Al₃B₄₈C₂ composite material produced according to the method that is the subject matter of the invention is a good alternative to the Al/B₄C composite material. This is because the ternary compound τ₃-Al₃B₄₈C₂, which forms the reinforcement, is in equilibrium with the Al matrix according to the literature. Furthermore, it has properties similar to those of B₄C, as can be noted by consulting the following table, and therefore constitutes a credible alternative to B₄C for producing a composite with ceramic matrix and reinforcement of the boron-rich carbide type.

Density Modulus Knoop Vickers Tenacity Thermal conductivity Compound (g · cm⁻³) (GPa) (GPa) (GPa) (MPa · m^(1/2)) (W · m⁻¹ · K⁻¹) Al₃B₄₈C₂ 2.62 — 23-37 25-30 4-5.3 19.6 (310 K) B₄C 2.52 450-470 39-37 38 3-4 30-42

According to the method that is the subject matter of the invention, the matrix and the reinforcement (and consequently the interface) are formed at high temperature and in situ, which has several advantages.

First of all, this dispenses with the difficulty related to the elimination of the oxide films present on the AlB₂ particles. The reactivity between AlB₂ and the carbon of the graphite crucible eliminates this oxide barrier that limits the wetting, adhesion and mechanical strength of the interface.

The reinforcements of the composite are obtained during the decomposition of the AlB₂ particles by germination/growth in the liquid phase. The matrix/reinforcement interface is therefore chemically clean (no impurities, oxides or the like) and therefore leads to optimum strength of the interface.

The reinforcements are formed in situ and have not had to undergo a grinding cycle, a grinding that is often liable to cause defects that are then initiation points for cracking of the composite material.

Moreover, the method that is the subject matter of the invention also has the advantage of simplicity of implementation. It makes it possible in particular to obtain directly a dense ingot having the internal geometry of the graphite crucible, since the formation of the ingot is done in the liquid state in the graphite crucible.

The potential applications of the method that is the subject matter of the invention are numerous. Mention can be made in particular of the fields requiring the production of lightened parts (parts for aeronautics (aeroplanes, helicopters, etc.), for automobiles, etc.). Mention can also be made of the fields that require the production of parts with high thermal conductivity through the presence of the aluminium matrix, but with a low coefficient of thermal expansion because of the presence of a high proportion of reinforcement particles. Such parts make it possible to discharge heat and have good dimensional stability and are therefore of interest for applications in the space sector or in power electronics. Finally, the production of lightweight parts comprising a high proportion of ceramic reinforcement having high hardness may also find an application in the ballistics sector for personal protection purposes.

Other features and advantages of the invention will emerge more clearly from a reading of the rest of the description that follows and which refers to the accompanying figure.

Naturally this additional description is given only by way of illustration of the invention and under no circumstances constitutes a limitation thereof.

BRIEF DESCRIPTION OF THE SINGLE FIGURE

The single FIGURE is an image obtained by scanning electron microscopy of an ingot obtained according to a first embodiment according to the method that is the subject matter of the invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

The method that is the subject matter of the invention is based on a so-called reactive synthesis method. This is because the matrix and the reinforcement of the composite material are obtained in situ by a reaction between two precursors. The precursors chosen are aluminium diboride (AlB₂) and graphite (C). AlB₂ is in the form of a powder and is placed in a crucible that is made from graphite. Preferably, the same graphite element, preferably a graphite piston, is used for compacting the powder and for hermetically closing the cavity of the crucible. The whole is then raised to high temperature. The heating is carried out at a temperature higher than the decomposition temperature of AlB₂, that is to say the temperature as from which there begins to be a liquid phase. In fact, at the decomposition temperature of AlB₂, that is to say 960° C., two phases are obtained, a liquid phase and a solid phase.

Preferably, the heating is carried out at a temperature of between 1000° C. and 1400° C., preferentially between 1200° C. and 1400° C., for a period which may be variable but which will generally be between 5 and 30 minutes. In fact, the duration of the heating at a given temperature is adjusted according to the microstructure that it is wished to obtain: the longer the heating period, the larger the size of the reinforcement particles.

Since the two AlB₂ and C phases are not in equilibrium, they react with each other in order to form Al and the mixed carbide Al₃B₄₈C₂.

Preferably, the temperature rises and falls are rapid, for the purpose of limiting both the size of the reinforcement particles and decomposition thereof during cooling.

At the end of the high-temperature synthesis, the graphite crucible can be eliminated by simple machining, then releasing the ingot of CMM composite material contained inside. Since the latter was obtained at a temperature higher than the melting point of Al, the presence of the matrix in the liquid state makes it possible to directly obtain a composite with a relative density greater than 99.5%.

We shall now produce a composite material Al/Al₃B₄₈C₂ according to the method that is the subject matter of the invention.

In a graphite crucible 8 mm in diameter, with a height of 5 mm and the walls of which have a thickness of 2 mm, 750 mg of aluminium diboride (AlB₂) powder is placed. The whole is heated to 1400° C. for 15 minutes. The heating ramp is approximately 340° C./minute, while cooling is obtained by soaking the crucible directly in an oil bath cooled to 0° C.

The microstructure of the Al/Al₃B₄₈C₂ composite thus obtained is observed under SEM (single figure). The white phase corresponds to the aluminium matrix and the black particles correspond to the reinforcement phase Al₃B₄₈C₂. It can be seen that the reinforcements are dispersed in the matrix homogeneously and have a size of between 200 nm and 5 μm (mean size approximately 700 nm).

The method that is the subject matter of the invention makes it possible to create an interface between a matrix and a reinforcement that is mechanically strong, but without leading to the decomposition of the reinforcement and to the creation of secondary phases that are detrimental to the properties of the composite. This is because, during the reactive synthesis between AlB₂ and the graphite (C), there are very few minor phases that are created and the composite therefore behaves essentially as a phase of Al (forming the matrix) and the phase of Al₃B₄₈C₂ (reinforcement), the minor phases being present in minimal quantities.

Finally, the method according to the invention provides a novel synthesis method for producing, in a simple manner and in quantity, composite materials with an Al matrix reinforced by particles of a mixed carbide of boron (B) and aluminium (Al), the properties of which are similar to those of a B₄C reinforcement.

REFERENCE CITED

[1] US Ser. No. 11/033,099 

What is claimed is:
 1. Method for manufacturing a part made from an Al/Al₃B₄₈C₂ composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al₃B₄₈C₂ are dispersed, said method comprising the following steps: a) placing a powder of chemical formula AlB₂ in a cavity of a graphite crucible; b) closing the cavity by means of a graphite element; c) heating the crucible to a temperature of at least 960° C. and less than or equal to 1400° C. in order to obtain formation of precipitates of mixed carbide of chemical formula Al₃B₄₈C₂ in liquid aluminium; d) cooling the crucible in order to solidify the liquid aluminium; e) removing the crucible; thereby the part made from Al/Al₃B₄₈C₂ composite material is obtained.
 2. Method according to claim 1, wherein the graphite element used to close the cavity is a graphite piston.
 3. Method according to claim 1, wherein the powder is placed in the crucible in a compressed form.
 4. Method according to claim 1, wherein the powder is placed in the crucible in a powdery form and step b) further comprises a compression of the powder.
 5. Method according to claim 4, wherein the compression of the powder and the closure of the cavity of the crucible are obtained by the use of a graphite piston.
 6. Method according to claim 1, wherein, at step c), the crucible is heated to a temperature ranging from 1000° C. to 1400° C. for a period ranging from 5 minutes to 30 minutes.
 7. Method according to claim 1, wherein the cooling at step d) comprises a temperature drop at a rate greater than or equal to 10° C./second until it reaches 600° C. 